JSS allows to define Jython routines to be executed as on-edit validator and post-function for every issue in a given JIRA instance. These are called Global Issue Edit Validation & Post-function.
You have to install patched versions of Issue Navigation (issuenav) and Quick Edit (quickedit) plugins for these routines to work. Patches change only OSGi/Spring manifests to allow subclassing and do not change any functionality.
Install the patched plugins via UPM, then restart your JIRA. After restart JSS Installer should show that both quickedit and issuenav support are enabled:
If you have an access to JIRA source files, you can build a patched plugins by yourself: